drm/i915: Calculate PSR register offsets from base + gen

Future generations will be changing these registers (thanks to design
for giving us an early heads up). To help abstract, create the
definition of the base of the register block, and define all registers
relative to that.

Design has promised to not change the offsets relative to the base.

v2: Also change IS_HASWELL checks to HAS_PSR

CC: Rodrigo Vivi <rodrigo.vivi@gmail.com>
CC: Intel GFX <intel-gfx@lists.freedesktop.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>